A sneeze, or sternutation, is a semi-autonomous, convulsive expulsion of air from the lungs through the nose and mouth, usually caused by foreign particles irritating the nasal mucosa. A sneeze expels air forcibly from the mouth and nose in an explosive, spasmodic involuntary action resulting chiefly from irritation of the nasal mucous membrane.[1] Sneezing is possibly linked to sudden exposure to bright light, sudden change (fall) in temperature, breeze of cold air, a particularly full stomach, or viral infection, and can lead to the spread of disease.

The function of sneezing is to expel mucus containing foreign particles or irritants and cleanse the nasal cavity. During a sneeze, the soft palate and palatine uvula depress while the back of the tongue elevates to partially close the passage to the mouth so that air ejected from the lungs may be expelled through the nose. Because the closing of the mouth is partial, a considerable amount of this air is usually also expelled from the mouth. The force and extent of the expulsion of the air through the nose varies.

Sneezing cannot occur during sleep due to REM atonia – a bodily state where motor neurons are not stimulated and reflex signals are not relayed to the brain. Sufficient external stimulants, however, may cause a person to wake from sleep to sneeze, but any sneezing occurring afterwards would take place with a partially awake status at minimum.[2]

Sneezing typically occurs when foreign particles or sufficient external stimulants pass through the nasal hairs to reach the nasal mucosa. This triggers the release of histamines, which irritate the nerve cells in the nose, resulting in signals being sent to the brain to initiate the sneeze through the trigeminal nerve network. The brain then relates this initial signal, activates the pharyngeal and tracheal muscles and creates a large opening of the nasal and oral cavities, resulting in a powerful release of air and bioparticles. The powerful nature of a sneeze is attributed to its involvement of numerous organs of the upper body – it is a reflexive response involving the face, throat, and chest muscles. Sneezing is also triggered by sinus nerve stimulation caused by nasal congestion and allergies.

The sneeze reflex involves contraction of a number of different muscles and muscle groups throughout the body, typically including the eyelids. The common suggestion that it is impossible to sneeze with one's eyes open is, however, inaccurate.[4] Other than irritating foreign particles, allergies or possible illness, another stimulus is sudden exposure to bright light – a condition known as photic sneeze reflex (PSR). Walking out of a dark building into sunshine may trigger PSR, or the ACHOO (autosomal dominant compulsive helio-ophthalmic outbursts of sneezing) syndrome as it's also called.[5] The tendency to sneeze upon exposure to bright light is an autosomal dominant trait and affects 18-35% of the human population.[6] A rarer trigger, observed in some individuals, is the fullness of the stomach immediately after a large meal. This is known as snatiation and is regarded as a medical disorder passed along genetically as an autosomal dominant trait.

It is known for some people to sneeze during the initial phases of sexual arousal. Doctors suspect that the phenomenon might arise from a case of crossed wires in the autonomic nervous system, which regulates a number of functions in the body, including "waking up" the genitals during arousal. The nose, like the genitals, contains erectile tissue.[medical citation needed]. This phenomenon may prepare the vomeronasal organ for increased detection of pheromones. Sneezing while menstruating may result in a sudden vaginal menses emission.[7]

While generally harmless in healthy individuals, sneezes spread disease through the infectiousaerosol droplets, commonly ranging from 0.5 to 5 µm. A sneeze can produce 40,000 droplets.[8] To reduce the possibility of thus spreading disease (such as the flu), one holds the forearm or the inside of the elbow in front of one's mouth and nose when sneezing. Using one's hand for that purpose has recently fallen into disuse [9] as it is considered inappropriate, since it promotes spreading germs through human contact (such as handshaking) or by commonly touched objects (most notably doorknobs).

Examples of preventive techniques are: the deep exhalation of the air in the lungs that would otherwise be used in the act of sneezing, holding the breath in while counting to ten or gently pinching the bridge of the nose for several seconds.

Proven methods to reduce sneezing generally advocate reducing interaction with irritants, such as keeping pets out of the house to avoid animal dander; ensuring the timely and continuous removal of dirt and dust particles through proper housekeeping; replacing filters for furnaces and air-handling units; air filtration devices and humidifiers; and staying away from industrial and agricultural zones. Some people, however, find sneezes to be pleasurable and would not want to prevent them.[10]

In Ancient Greece, sneezes were believed to be prophetic signs from the gods. In 401 BC, for instance, the Athenian general Xenophon gave a speech exhorting his fellow soldiers to fight against the Persians. A soldier underscored his conclusion with a sneeze. Thinking that this sneeze was a favorable sign from the gods, the soldiers were impressed.[11] Another divine moment of sneezing for the Greeks occurs in the story of Odysseus. His waiting wife Penelope, hearing Odysseus may be alive, says that he and his son would take revenge on the suitors if he were to return. At that moment, their son sneezes loudly and Penelope laughs with joy, reassured that it is a sign from the gods (Odyssey 17: 541-550). It may be because this belief survived through the centuries, that in certain parts of Greece today, when someone is asserting something and the listener sneezes promptly at the end of the assertion, the former responds "bless you and I am speaking the truth", or "bless you and here is the truth", "γεια σου κι αλήθεια λέω", ya sou ki alithia leo, or "γεια σου και να κι η αλήθεια", ya sou ke na ki i alithia). A similar practice is also followed in India.[12][13] If either the person just having made a not most obvious statement in Flemish, or some listener sneezes, often one of the listeneners will say " 't is beniesd", literally "It's sneezed upon", as if a proof of truth – usually self-ironically recalling this old superstitious habit, without either suggesting doubt or intending an actual confirmation, but making any apology by the sneezer for the interruption superfluous as the remark is received by smiles.[14]

In Europe, principally around the early Middle Ages, it was believed that one's life was in fact tied to one's breath – a belief reflected in the word "expire" (originally meaning "to exhale") gaining the additional meaning of "to come to an end" or "to die". This connection, coupled with the significant amount of breath expelled from the body during a sneeze, had likely led people to believe that sneezing could easily be fatal.[citation needed] Such a theory could explain the reasoning behind the traditional English phrase, "God bless you", in response to a sneeze, the origins of which are not entirely clear (see "Traditional Responses To A Sneeze" below for alternative explanations). Sir Raymond Henry Payne Crawfurd, for instance, the registrar of the Royal College of Physicians, in his 1909 book, "The Last Days of Charles II", states that, when the controversial monarch was on his deathbed, his medical attendants administered a concoction of cowslips and extract of ammonia to promote sneezing.[15] However, it is not known if this promotion of sneezing was done to hasten his death (as coup de grace), or as an ultimate attempt at treatment.

In certain parts of Eastern Asia, particularly in Chinese culture, Korean culture, Japanese culture and Vietnamese culture, a sneeze without an obvious cause was generally perceived as a sign that someone was talking about the sneezer at that very moment. This can be seen in the Book of Songs (a collection of Chinese poems)[16] in ancient China as early as 1000 BC, and this belief is still depicted in present-day manga and anime. In China, Vietnam, South Korea, and Japan, for instance, there is a superstition that if talking behind someone's back causes the person being talked about to sneeze; as such, the sneezer can tell if something good is being said (one sneeze), something bad is being said (two sneezes in a row), even if someone is in love with them (three sneezes in a row) or if this is a sign that they are about to catch a cold (multiple sneezes).

Parallel beliefs are known to exist around the world, particularly in contemporary Greek, Slavic, Celtic, English, French, and Indian cultures. Similarly, in Nepal, sneezers are believed to be remembered by someone at that particular moment.

In Indian culture, especially in northern parts of India, Bengali (Bangladesh and Bengal of India) culture and also in Iran, it has been a common superstition that a sneeze taking place before the start of any work was a sign of impending bad interruption. It was thus customary to pause in order to drink water or break any work rhythm before resuming the job at hand in order to prevent any misfortune from occurring.

Contrarily, in Polish culture, especially in the Kresy Wschodnie borderlands, a popular belief persists that sneezes may be an inauspicious sign that one's mother-in-law speaks ill of their son-in-law at that moment. The same phenomenon is thought to correspond to daughters-in-law and their mothers-in-law. As with other Catholic countries, such as Mexico, Italy, or Ireland, the remnants of pagan culture are fostered in Polish peasant idiosyncratic superstitions.

When one of you sneezes, let him say, "Al-hamdu-Lillah" (Praise be to God), and let his brother or companion say to him, "Yarhamuk Allah" (May God have mercy on you). If he says, "Yarhamuk-Allah", then let [the sneezer] say, "Yahdeekum Allah wa yuslihu baalakum" (May God guide you and rectify your condition).

In English-speaking countries, one common verbal response to another person's sneeze is, "[May God] bless you". Another common verbal response to another's sneeze is, "Gesundheit"; Gesundheit being a German word that means, appropriately, "good health".

Several hypotheses exist for why the custom arose of saying "bless you" or "God bless you" in the context of sneezing:

Some say it came into use during the plague pandemics of the 14th century. Blessing the individual after showing such a symptom was thought to prevent possible impending death due to the lethal disease.[citation needed]

In Renaissance times a superstition was formed claiming one's heart stopped for a very brief moment during the sneeze, saying bless you was a sign of prayer that the heart wouldn't fail.

It has also been stated that one says "(God) bless you" so that one does not catch the flu, cold, or any other forms of sickness.

Other cultures have similar traditions:

In Muslim countries, after a person sneezes they often say Al-ḥamdu lillāh (Arabic: الحمد لله) is an Arabic phrase meaning "Praise to God". His/her companion should say to he/she Yarhamuk-Allah "May Allah have mercy on you." The sneezing person should say Yahdikum-ullah wa yuslihu balakum "May Allah guide you and render sound your state of affairs." [17]

In Iran, it is common to respond to sneezing with the Persian phrase "عافیت باشه", which translates to "health", similar to common European expressions.

Indian culture is to respond with Krishna, similar to a blessing in western cultures.

In Slovakia, after a person sneezes, it is proper to say "Na zdravie!" which means "For health!"; a proper response should be "Ďakujem" which means "Thanks". This is also the case in Finland, where "terveydeksi" means "for health".

In Turkey, after a person sneezes, it is proper to say "Çok yaşa" which means "Live long"; a proper response should be "Sen de gör" which means "May you see too [that I lived long enough]".

In Telugu, a reciprocation to someone's sneeze is "chiranjeeva sataish" (చిర౦జీవ) which means "may you live long" (from Sanskrit).

In Tamil, a reciprocation to someone's sneeze is "Dheergaiyish" which means "may you live long" (from Sanskrit).

In Japanese entertainment, a character's sneeze frequently means that someone elsewhere is talking about said character by coincidence.

Sneezing is not confined to humans or even mammals. Many animals including cats,[19] dogs,[20] chickens[21] and iguanas[22] sneeze. African wild dogs use sneezing as a form of communication, especially when considering a consensus in a pack on whether or not to hunt.[23]